Biology Reference
In-Depth Information
radiosensitivity seen in isogenic cell pairs with or
without functional BRCA1/2 gene are too small to be
detectable in a clinical population that has considerable
variations in tumor radiosensitivity due to other genetic
or microenvironmental factors.
Do defects in components of the FA pathway confer
clinical radiosensitivity? There are several reports on
increased normal tissue toxicity in patients with FAwho
received radiation as part of their conditioning regimen
prior to bone marrow transplantation
284,285
or as part of
therapy for solid cancers.
286
e
288
By implication, cancers
arising in FA patients would be expected to be similarly
hypersensitive to radiation, although due to the small
number of patients reported in the literature direct
evidence is lacking. Is the mechanism underlying this
radiosensitivity related to a presumed HRR defect result-
ing from lack of FANCD2 activation? Strikingly, disrup-
tion of the function of FANCD2 or other FA proteins
generally does not lead to cellular hypersensitivity to IR
in vitro
,
287,289
e
295
perhaps arguing against an underlying
DSB repair defect. Notably, the radiation hypersensitivity
of fibroblasts from the skin of FA patients is unmasked
when cells were irradiated under conditions of 0
e
3%
oxygen.
295,296
The ability of FANCD2-mutant cells to
form IR-induced RAD51 foci was similar to FANCD2
wild-type cells, independent of oxygen concentration,
arguing against variations in HRR.
295
It is important to
recognize that most human tissues including skin are
not exposed to the artificial 20% oxygen concentration
that is commonly used in cell culture experiments, but
typically are in an environment of approximately 3%
oxygen.
297
This is of potential relevance for the described
discrepancy between clinical and cellular FA radiosensi-
tivity.
286,287,289
The reasons for this discrepancy remain
unknown but one could speculate that FA cells may
have evolved tolerance mechanisms under conditions
of high oxygen tension
in vitro
that allow them to survive
IR-induced DNA breaks and that can be reversed upon
lowering of oxygen concentration. These observations
are likely related to the known, yet unexplained oxygen
hypersensitivity of FA cells. More studies will be needed
for a better understanding of the radiosensitivity pheno-
type conferred by FA disruption, which will be impor-
tant for a prediction of radiosensitivity of FA-deficient
cancers in a clinical setting.
Are there types of radiation damage that are more
dependent upon HRR than others? There is increasing
interest in the use of proton and carbon ion radiation
for the treatment of adult and childhood cancers.
298
Compared to photon IR, high-LET particle radiations
generate an increased frequency of clustered DNA
damages that are difficult to repair.
264,299
e
301
Studies
with HRR-deficient mutants have suggested that unre-
paired clustered lesions will collide with replication
forks in S-phase, thus triggering HRR.
302,303
possible that for a fraction of spatially correlated breaks
induced by high-LET radiation, extensive resection of
the DSB ends to generate appropriate overhangs for
HRR would be a more feasible way to repair these
lesions than NHEJ.
302
Proton beam radiation has vari-
able LET depending on beam energy, which somewhat
complicates the interpretation of experimental findings.
Generally, proton beam energies used in the clinic
(~50-250 MeV) are associated with low LET and thus
a relatively low frequency of clustered damages and
relative biological effectiveness. Thus, whether low-
LET proton beam radiation causes DNA damages that
are more dependent on HRR for repair than for photon
IR remains to be established. Interestingly, Rostek
et al.
304
showed in a yeast system that 250 MeV protons
caused DNA damages that were preferentially repaired
by HRR and post-replication repair pathways. The
caveat of this system is that in yeast HRR contributes
much more to DSB repair than in mammalian cells
where NHEJ is most important for IR-induced DSB.
The potential clinical significance of the findings above
is that tumors with known HRR defects may be particu-
larly suitable for proton beam or other particle radiation
treatments.
In summary, there is preclinical evidence that IR may
be more effective in HRR deficient tumors compared to
proficient tumors, though convincing clinical support
for this concept is still lacking. The use of particle radia-
tion rather than low LET photon IR may be better suited
to exploit HRR defects in this regard. In addition, for
clinical application, combining IR with molecular tar-
geted agents or DNA damaging chemotherapies known
to be selectively toxic to HRR-deficient tumors should be
most beneficial.
Modifying IR-Induced DNA Damage
Each Gy of low-LET IR causes ~ 1,000 SSB and
2,000
e
3,000 base damages per cell.
261
It has been gener-
ally held that these lesions do not contribute signifi-
cantly to radiation-induced kill of repair-proficient
cells.
305
However, disruption of the repair pathways
responsible for the removal of these lesions may confer
cellular radiosensitivity, especially in HRR-deficient
cells. It is thought that oxidative base damage is con-
verted to SSB intermediates during base excision repair
(BER), while directly induced SSB require further end-
processing. Subsequent SSB repair (SSBR) involves
PARP-1, XRCC1, polymerase
b
, and DNA ligase III
(reviewed in
306
). A smaller portion of SSBR involves
gap filling after loss of multiple nucleotides and requires
additional enzymes (long patch repair). Importantly,
work from Helleday and colleagues has demonstrated
that PARP is not essential for BER/SSBR but rather
that PARP coordinates and enhances the rate of repair
of these lesions.
307
In addition, PARP has a role at least
It is also
